1. Field of the Invention
The present invention generally relates to a magnetic brush development type developing apparatus, an image forming unit and an image forming apparatus.
2. Description of the Related Art
Conventionally, a magnetic brush development type developing apparatus develops a latent image on a photoconductor in the following process. A developing sleeve, which is disposed in an aperture of a developing unit casing, serves as a developer carrying member to form a magnetic field with a permanent magnet thereof. This magnetic field forms a magnetic brush of a two-component developer containing a toner and a carrier. The latent image is developed by the magnetic brush between the photoconductor, which serves as an image carrying member, and the developing sleeve.
FIG. 1 is a front view of a conventional magnetic brush development type developing apparatus for performing the above process. Referring to FIG. 1, a developing sleeve 101 is disposed to closely face a drum-shaped photoconductor 102. The developing sleeve 101 is rotated clockwise, and, in contrast, the photoconductor 102 is rotated counterclockwise. For example, six permanent magnets, three of which are illustrated in FIG. 1 and designated as 103a, 103b, and 103c, respectively, are fixed in a radial pattern in the interior of the developing sleeve 101. Here, the permanent magnet 103a, which faces the photoconductor 102 as illustrated, is called a main pole. When the permanent magnet 103a creates a magnetic field, the magnetic field forms a magnetic brush of a two-component developer whose components are a toner and a magnetic carrier. The magnetic brush develops a latent image on the photoconductor 102. The permanent magnets other than the main pole permanent magnet 103a are called auxiliary poles and are used to form adequate magnetic brushes at the main pole. The developing sleeve 101 is accommodated in a developing unit casing 105. The developing unit casing 105 has an aperture 104 at a position opposite to the photoconductor 102 such that the circumferential surface of the developing sleeve 101 is partially exposed through the aperture 104. As shown in FIG. 1, developer carrying screws 106 and 107 are disposed at the bottom of the developing unit casing 105 such that the developer carrying screws 106 and 107 are located below the developing sleeve 101. The developer carrying screws 106 and 107 work to deliver toners supplied from a toner supplying part, which is not illustrated, to the circumferential surface of the developing sleeve 101 and mix collected toners and supplied developers. Additionally, a doctor blade 108 is provided in the developing unit casing 105 as illustrated. The doctor blade 108 works to maintain the am t of the developer provided to the photoconductor 102 by each magnetic brush at a constant amount.
According to such a conventional magnetic brush development type developing apparatus, however, there is a risk that the toner and the carrier may easily splash from the aperture 104 of the developing unit casing 105. In this case, most of the toner and the carrier fall under the developing apparatus. If the developing apparatus is accommodated in an image forming apparatus such as a copier and a printer, the toner and the carrier are accumulated in the interior of the image forming apparatus and make the interior unclean. In this condition, there are some problems. For instance, if an operator tries to eliminate a paper jam, the operator is likely to dirty his/her hands with the toner or the carrier accumulated in the image forming apparatus. Also, if the toner and the carrier are attached to inner electric parts of the image forming apparatus, there is a risk of noise occurrence. Additionally, if the image forming apparatus is a tandem type full-color image forming apparatus, which has an intermediate transfer belt as a second image carrying member below the photoconductor 102 and the developing apparatus, there is a risk that the splashing toner and carrier fall on the intermediate transfer belt or a transfer paper carried by the intermediate transfer belt.
In order to eliminate the above-mentioned problems, some related art applications are presented below.
Japanese Laid-Open Patent Application No. 05-119626 discloses a developing device that incorporates filters for preventing toner splashes at both ends of an aperture of a developing unit casing thereof.
Additionally, Japanese Laid-Open Patent Application No. 10-026861 discloses a technique that makes use of a phenomenon that splashing of toner and carrier occurs in a contact region (which is referred to as a nip part hereinafter) between a photoconductor and a magnetic brush formed on a developing sleeve. According to the disclosure, carrier collecting means are disposed at a position below the nip part such that the carrier collecting means are separated from the photoconductor and the developing sleeve at respective predetermined intervals.
Additionally, Japanese Laid-Open Patent Application No. 10-247042 discloses a technique for collecting splashing toner and splashing carrier by using airflow caused in a developing apparatus. According to the disclosure, airflow generation means are disposed between a photoconductor and carrier collecting means so as to cause an air to flow from the exterior to the interior of the developing apparatus.
However, the above-mentioned conventional developing apparatuses have some problems. For instance, if the filter or the carrier collecting means are disposed only at ends of a developing unit casing or only at a portion of a developing sleeve so as to prevent the toner splash, toner and carrier can still splash due to airflow from the interior of the developing unit casing, that is, outward airflow. Thus, the above-mentioned conventional techniques cannot sufficiently overcome the toner and carrier splash problems.
In particular, as the conventional developing apparatus operates at higher speeds and the size thereof becomes smaller, the outward airflow becomes more problematic. The generation mechanism of the outward airflow has not been clearly explained. In order to prevent the toner and carrier splash, it is indispensable to demonstrate the generation mechanism and design a developing apparatus that can control the entrance and the exit of the airflow.
Herein, the above-mentioned toner splash problem is investigated in more detail. Although it is known that the toner splash problem is caused by nonuniform developer characteristics, for instance, nonuniformity of the Q/M value where Q represents an amount of electrostatic charge and M represents a toner mass, toner also splashes due to the physical design of the developing apparatus.
A part of the developer carried by the developing sleeve 101 escapes from an electric attractive force with a latent image or a carrier on the photoconductor 102 due to physical contact with the photoconductor 102 and the up-and-down motion of magnetic brushes, and then the escaping developer floats on the airflow in the image forming apparatus.
While the photoconductor 102 and the developing sleeve 101 are rotated, an accompanying airflow arises on the boundary between the surfaces thereof, resulting in the toner being carried along by the airflow. The accompanying airflow hits the developing unit casing 105 at a downstream position with respect to the rotational direction of the developing sleeve 101, or a portion of the accompanying airflow flows out of the developing apparatus there. The separated airflow becomes the above-mentioned outward airflow and causes the floating toner to splash out of the developing unit casing 105. The above-mentioned toner splash mechanism is referred to as the first factor hereinafter. For instance, the above-mentioned developing apparatus according to Japanese Laid-Open Patent Application No. 10-247042 challenges only the first factor.
In addition, there is another factor that has not been conventionally taken into account. This factor focuses attention on viscosity of the air, which cannot be neglected in a small region where a magnetic brush is formed. A mass of air is trapped in an inner space in a plurality of magnetic brushes, and the trapped air passes through a narrow gap between the developing sleeve 101 and the developing unit casing 105. The narrow gap is referred to as a casing gap hereinafter. Although the developer is collected by the developing unit casing 105, the trapped air hits the developing unit casing 105 and is likely to physically cause the toner to flake away. Consequently, the outward airflow is generated from the casing gap. This toner splash mechanism is referred to as the second factor hereinafter.
When the first factor and the second factor are combined, an abnormal outward airflow is generated at the downstream side of the developing unit casing 105. Namely, the interaction between the behavior of magnetic brushes and the portion of the developing unit casing 105 causes the outward airflow from the developing unit casing 105.
Furthermore, the toner splash mechanism due to the second factor is investigated below with reference to FIG. 2 and FIG. 3 in more detail.
FIG. 2 is an enlarged view of a developing part of the conventional magnetic brush development type developing apparatus in FIG. 1. This conventional developing apparatus uses a two-component developer. Referring to FIG. 2, when the permanent magnet 103a, which serves as the main pole, is fixed at a position corresponding to the aperture 104 of the developing unit casing 105, a magnetic field due to the main pole forms a magnetic brush 110 of the two-component developer 109. When a latent image on the photoconductor 102 is in contact with the magnetic brush 110, the latent image is developed.
As previously mentioned, toner has the tendency to splash more frequently at the downstream side of the nip part with respect to the rotational direction of the developing sleeve 101.
FIG. 3 is an enlarged view of the downstream side of the nip part wherein the gap between the developing sleeve 101 and the developing unit casing 105 and the height of a magnetic brush 111 are highlighted. In general, the developing apparatus includes the permanent magnets 103b,103c as the auxiliary poles to maintain the magnetic field in the vicinity of the main pole of the permanent magnet 103a that forms the magnetic brush 110 in the contact point between the photoconductor 102 and the developing sleeve 101. As a result, magnetic brushes 111 are also formed at positions corresponding to the auxiliary poles on the developing sleeve 101. However, since the developing unit casing 105, which has the aperture 104 as an end part thereof, is located around the permanent magnet 103b at the downstream position of the nip part, the magnetic brush 111 and the developing unit casing 105 can physically interact, for example, a collision can occur between the magnetic brush 111 and the developing unit casing 105.
Referring to FIG. 3, an amount of two-component developer 109 prescribed by the doctor blade 108 is accumulated on the circumferential surface of the developing sleeve 101, resulting in forming the magnetic brush 111 due to the permanent magnet 103b. At this time, an air layer 112 of a predetermined height H is trapped in the inner space of the magnetic brush 111 and then is carried to an area 113 on the developing unit casing 105. In particular, if the magnetic brush 111 is formed, a greater amount of air is trapped therein. This phenomenon is caused because the two-component developer 109 includes an extremely fine carrier whose grain diameter is dozens of micrometers and the viscosity of the air has considerable influence. Since the developing unit casing 105 is closed at the inner side from the area 113 and has no outlet for the air, the trapped air has nowhere to go and is released to the exterior of the developing unit casing 105 through the upper side of the magnetic brush 111 as illustrated. The released air becomes an outward airflow 114. The outward airflow 114 can be observed by means of a high-speed camera and an image processing technique.
The outward airflow 114 has the most significant influence on the toner and carrier splash. When the magnetic brush 111 collides with the developing unit casing 105 or the developer carrying screws 106 and 107 mix the two-component developer 109 in the developing unit casing 105, the outward airflow 114 directly blows out floating toner and carrier from the developing unit casing 105.
By using image measuring means to make visible the outward airflow 114, it was observed that the boundary layer (the air layer 112) has the following thicknesses under the conditions as follows;                thickness with the two-component developer 109: 0.07 (mm)        thickness without two-component developer 109: 0.2 (mm)        condition:                    diameter of the developing sleeve 101: 60 (mm)            rotational speed of developing sleeve 101: 5 (rad/s)            angle between the nip part and the permanent magnet 103b: π/4 (rad)                        
The above measurement result shows that the two-component developer 109 traps air on the surface of the developing sleeve 101. In order to eliminate the toner and carrier splash problem, it is necessary to improve such a structure of the developing apparatus where the outward airflow 114 is generated.